4 Barrel setup stumbles in 3rd and 4th.

[jftl]

Hello all,

Been a while since I've been on here. Recently moved "Joe'd" (our 1965 Corvair Corsa) from my dad's house to mine so it will start getting a little more TLC. This car is the non-turbo corsa, and has the 4 barrel conversion setup with a 390 cfm holley 4 barrel. The car does ok at idle (though we set it a little high), running through first and second is no problem. When you get into 3rd and 4th, if the rpms aren't very high the car will stutter/hesitate with moderate accelerator pressure, especially in 4th gear. Here's a scenario to explain what I mean. Starting from a stop, run through the gears like normal, get up to 4th, and if you try to give it more gas the engine will cough and bog down for a second until you let off. If you use very light/gradual accelerator pressure it doesn't happen.

The car still has the original points set up, and I'm considering going to the Crane ignition that Clarks sells. The carb has been rebuilt, but it is a 4 barrel, so I don't know if the car is just getting more fuel then it can handle or what. Plugs and plug wires have not been changed by us as of yet (had the car about a year), and we haven't messed with timing or anything like that.

Thanks!
Kevin

[flat6_musik]

Is your vacuum advance hooked up? If so, disconnect it and try another run. I've seen some weird things happen when that breaker plate moves (when the vacuum advance starts to pull it). Yes, I'd also recommend getting an electronic ignition setup and maybe even a rebuilt distributor right off the bat. But having said all this, I have a sneaking suspicion that the secondary side of that 390 may be not delivering fuel correctly OR you're just experiencing the usual "center 4 barrel driveability suckiness" Is it fully warmed up? Is the outside air very cold??

[jftl]

Hi Flat 6,

It is, so I'll try that next time I drive it. Thanks!

It will do it even fully warmed up, although it does get alot better the warmer it is. I live in AZ, so it generally doesn't stay cold long =). My temp gauge is another thing I need to look into. It rarely gets above around 250 (I'll confess I don't know where "normal" is yet, my first air cooled after all).

4 Barrel setups tend to drive poorly eh? The car came with it, so we really didn't have anything to compare it to (or the old parts to put back in). Is there anything that helps make them drive better this way, or is going back to 4 1 barrels my only option (likely not very cost effective). I really want to make this a cool "jump in it and cruise" car, so good driveability is pretty high on my list.

Thanks!
Kevin

[Motornoggin]

Sounds like it is too lean. Try jetting it slightly richer and see what that does.

[jftl]

Ran "Joe'd" last night with the vacuum advance disconnected, and the car was throughly warmed up. Seemed to make quite a difference, so I may run it that way for now.

Motornoggin= Thanks for the advice, will look into doing that. Probably will do the ignition etc first since we don't know the condition of those parts and go from there. I have a feeling getting this to run well is going to be a long process lol.

-foot

[UNSAFE]

Okay okay - I know some people claim success with the centermount 4 barrel but it didn't work for me when I tried it.

In theory it might be good for some added WOT performance but the basic principle seems to be a compromise to me.

First off having the throttle plates so far away from the intake valves is doomed from the git go. When you floor the pedal the engine needs a richened mixture right away but it has to clear the already lean mixture out of the carb plenum and the long tubes and the plenum on the head before it can get the rich mixture that it needs after you floored it. This takes time and time causes a lag before anything happens.

Also the fuel is not really mixed with the air as it goes down the intakes tubes towards the engine. The air is merely carrying the fuel with it. Since the fuel is heavier than the air that's carrying it can not accelerate as fast as the air and it starts to precipitate ( fall as liquid and form droplets in the manifold runners.)

They do look cool tho !

[jftl]

Hi Unsafe,

Yeah, I can see the inherent flaws in it's design like you say. Since it's what we have, we'll likely try and make it work for now. My ultimate goal with the car is to have it be a nice cruising car, so performance isn't as much of an issue (at least right now). All the other stuff we plan to change will work well with this, or any other carb setup, so if we change to something else later it will be fine.

Thanks for the input!

Kevin

[65Ragtop]

Hey Kevin. The center 4 barrel intake is not a great design and the double "plenum, ram tube, plenum" layout does indeed encourage precipitation of the fuel from the mixture, mainly in the second plenum where the mixture slows down, expands and looses energy. I have stated my opinion on this type of intake on other threads here so having said that........The center mount 4 barrel can work just fine if you can get it dialed in properly, there is just not much of a performance improvement to be had. In my opinion you are just trading one set of issues (carb balance, sloppy linkage issues) with the 4 carb set up. for a second set of issues with tuning and cold weather grumpyness with the centermount set up. And, I do think they look cool, especially with a really nice air cleaner set up!

The 390 Holly is a good size carb if you are going to run this for awhile, some don't like it but it can work out well if you can get it set up and jetted right.

I think you starting with the ignition improvements is a great plan. It is a semi weak spot in the engine department (it was fine in it's day for the daily driver). If you go with the Crane unit, put the controler in a protected area, I hear they may be heat sensitive. Get your ignition set up and work on tuning the car and I think you will have a good running engine to start driving around town.

[jftl]

Hi Ragtop, thanks for the tips/opinions. Looks like that's exactly what we are going to do. =)

After removing the windshield trim over the weekend I decided to try bumping the base idle adjustment down some. After warming up it acutally made the problem some better. Guess we'll see how it does after the ignition system is re-done.
One problem I'm having is the accelerator linkage isn't allowing it to go all the way back down to idle when you let off the gas. If you push on the linkage, or tap on the accelerator again that it will go down to where I set it, so I'm probably going to have to find away to get it to return properly everytime.

-Kevin

[Greybeard]

If an engine runs good at full throttle in 1st and 2nd, and then starts missing in 3rd or 4th, I'd look at full delivery. Electrical generally is RPM related and should rear it's head in all gears.
When it lays down and surges only in the upper gears, the float bowl is running dry, leaning it out. You should have a small bronze filter in the carb inlet, should also have and inline filter. The first thing I would do is change them. Also look for kinked hoses and lines. The last place to look will be the fuel pump.
Good luck

[jftl]

Greybeard,

Not sure on the small bronze filter, but the inline filter has been changed already. I'll check the fuel lines next time I'm under the car.

Thanks!
-Kevin

[65 Topless]

ok, I just ran across this post. Has anything changed since your last post Let me know you can contact me either here or at dudakt@gmail.com I have this setup and have some advise for you if you plan to continue to run this setup. sort of an alternative. Please let me know. Tom 65 Corsa Vert 140/4bbl/4spd

[jftl]

Hey Tom,

We do still have this setup. Just got the carb reworked after it sprang a major leak from the accelerator pump. Haven't got it back on yet (hope to this evening), but really hoping it will make an over all improvment (the guy that fixed it also did some tinkering with the carb, and replaced some parts that were bad). Would still love to hear your advise.

Thanks and God bless,
-Kevin